doc: typo: s/can not/cannot/ and s/is does/does/

doc: typo: s/can not/cannot/ and s/is does/does/

"Can not" suggests one has the option to not do something, whereas
"cannot" more strongly suggests something is disallowed or impossible.

Noticed "can not", mistakenly used instead of "cannot" in git help
glossary, then ran git grep 'can not' and found many other instances.
Only files in the Documentation folder were modified.

'Can not' also occurs in some source code comments and some test
assertion messages, and there is an error message and translation "can
not move directory into itself" which I may fix and submit separately
from the documentation change.

Also noticed and fixed "is does" in git help fetch, but there are no
other occurrences of that typo according to git grep.

Signed-off-by: Mark Rushakoff <mark.rushakoff@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>